Ionization gauges and method of operation thereof



April 28, 1959 J. M. LAFFERTY IONIZATION GAUGES AND METHOD OF' OPERATIONTHEREOF Filed Oct. 17, 1957 myn/Emana f77 Venter: dames /Vl La'efty, by7Qv/ d United States Patent O IONFZATION GAUGES AND METHOD ,0F OPERATIONTHEREOF James M. Latferty, Schenectady, NX., assignor to GeneralElectric Company, a corporation of New York Application October 17,1957, Serial No. l690,848

8 Claims. (Cl. 3137) The present invention relates to improvedionization gauges and methods of operating the same to measure extremelylow gas pressures.

ionization gauges are vacuum discharge devices generally including athermionic cathods, an anode and a collector electrode. Thermallyejected electrons pass from the cathode to the anode. lf a gas ispresent, these electrons may undergo ionizing collisions with gasmolecules. At low pressures, the probability of such collisions isproportional to the number of gas molecules present and, hence, to thegas pressure. Accordingly, in an ionization gauge, the positive ionscreated by such collisions are -collected by a negatively biasedcollector electrode. Collector electrode current is then a measure ofgas pressure.

It has been found that if the electron paths are increased in length thenumber of collisions per electron may be increased, thus increasing thesensitivity of the gauge. In magnetron ionization gauges, van axialmagnetic eld is imposed, causing electrons to spiral in thecathode-anode space, and at cut-Dif, just miss the anode and return tothe cathode region. Magnetron ionization gauges operating upon thisprinciple exhibit greatly irnproved sensitivity.

It has been found, however, that even `present-day magnetron ionizationgauges may not be used to measure gas pressures lower than l-8 mm. Thisis due primarily to photoelectric emission by the collector electrodedue to bombardment by soft X-rays. These soft X-rays are emitted at theanode when high velocity electrons are incident thereupon. Sincephotoelectric emission by the collector electrode is indistinguishablefrom positive ion collection, this phenomenon establishes an etfectivelower limit to the pressure which may lbe measured by presentdayionization gauges.

Accordingly, it is an object of the present invention to provideimproved ionization gauges and methods of operation capable ofmeasuring, heretofore unobtainable, low pressures.

A further object is to provide a method of operating ionization gaugeswhich greatly decreases background currents.

Another object of the present invention is to provide improvedionization gauges of such size and construction to enable them tofunction effectively at extremely high temperatures.

In accord with the present invention, pressures as low as approximatelyl013 mm. of mercury are measured utilizing a magnetron ionization gaugehaving a thermionic cathode, van anode and a negatively biased collectorelectrode. The value of applied axial magnetic eld and cathode-anodepotential 'are adjusted so that the electrons emitted by the cathodejust miss the anode and the tube operates under cut-soif conditions. Toreduce background currents, the thermionic cathode is operated at a muchlower temperature than heretofore utilized, and electron current isreduced a factor of 104 of that` convention'ally used in ionizationgauges. At these 4low cur- 2,884,550 Patented Apr. 28, 1959 l2 rents the-amount of soft X-rays emitted at the `anode is markedly decreased,decreasing the background photoemission by the collector electrode andlowering the minimum pressures measurable by the device.

This method maybe practiced utilizing most magnetron ionization gauges.Most prior art gauges, however, -are not suitably protected againstleakage currents which would prevent lowering of the measurable currentminimum, as above. Accordingly, I further provide shielding means andhigh leakage resistances which reduce leakage currents to a levelconsistent with the vimprovements of the foregoing method.

In tubes operated and constructed as above, it is essential that anodeand cathode be precisely located and so mounted as to maintain theirposition in spite of thermal and mechanical stress in order that highsensitivity be maintained. Accordingly, I provide anode and cathodestructure ideally suited to the maintenance of high sensitivity in spiteof thermal land mechanical stress.

The noval features believed characteristic of the present invention areset forth in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood byreference to the following detailed description taken in connection withthe accompanying drawing in which:

Fig.l 1 illustrates, in vertical cross-section, Ia magnetron ionizationgauge constructed in accord with one feature of the present invention,

Fig. 2 illustrates, in vertical cross-section, another magnen-onionization gauge constructed in accord with another feature ofthepresent invention, yand Fig. 3 is a set of curves illustrating electronVand positive ion currents in magnetron ionization gauges.

The ionization gauge of Fig. l includes yan anode cylinder 1 supportedin place by an integral annular concentric anode support member 1', alamentary thermionic cathode 2 extending lalong the longitudinal aXis ofyanode cylinder 1, `a disk-like collector electrode 3 and a meshcollector kelectrode 4, supported by annular collector electrode supportmember 4, at opposite ends of anode 1 and exterior thereof andperpendicular to the longitudinal axis thereof. Means for supplying anaxial magnet lield comprises an annularcylindrical magnet 5 which maylbe either an electromagnetic coil or a permanent magnet.

The ionization gauge of Fig. 2 also includes a cylindrical anode 6, adoubled lamentary thermionic cathode 7 extending along the longitudinalaxis of anode cylinder v6, a disk-like collector electrode 8 exterior ofanode cylinder 6 at one vend thereof and Vperpendicular to thelongitudinal `axial thereof. Means for supplying an axial magnetic eldcomprises an annular cylindrical magnet f9 which may be either apermanent magnet or an ,electromagnetic coil.

In bothvof the magnetron ionization gauges as illustrated in Figs. 1 and2, electrons are thermionically ejected from the cathode and migrate tothe anode. During the passage from cathode to anode a fraction of thegaseous molecules present within the device are ionized by collisionswith the migrating electrons. A fraction of the positive ions created bysuch collisions are collected at the negatively biased collectorelectrode causing a current to llow in the collector electrode circuit.At the `low pressures at which ionization gauges are utilized, thispositive ion vcurrent is proportional to the pressure of the gas withinthe gauge. For increased sensitivity, that is, for amaximum positive ioncurrent per unit electron current, .the length ofthepath ofthe electrons`between cathodeand anode should befa maximum. Electronspassingfromcathode yto anode lundergo a spiral motion'caused by the passage of.a Vcharged particle in a magnetic field. The spiral pathgtaken by theelectronsis thus longer than the straight path would be in the absenceof a magnetic field. Maximum sensitivity is obtained in a magnetronionization gauge when the magnetic field is increased to a degree thatelectrons emitted from the cathode and accelerated under the electricfield are curved to such an extent that they just fail to impinge uponthe anode and curve back to the cathode region. When the magnetic fieldis of this order, the device is essentially cut ofi and substantially noelectron current flows between cathode and anode. Under this condition,the path length of the electrons through the gas filled region betweenthe cathode and the anode is a maximum and a maximum number ofionization collisions occur for each electron passing within this space.

In Fig. 3 of the drawing, the increase in ion current achieved byoperating a magnetron ionization gauge at cut olf conditions is showngraphically. In Fig. 3, electron current (curve A) and positive ioncurrent (curve B) are plotted as a function of the applied magnetic eld.The positive ion current is very low initially at low magnetic fieldswhen the electrons ow radially from the filament to the cathode. As themagnetic eld is increased and electrons begin to miss the anode, the ioncurrent rises sharply to a maximum yat cutoff. As the magnetic field isfurther increased, sensitivity drops and then decreases more slowly. Thedrop with increased magnetic field is due to a further constriction ofthe electron paths causing them to gyrate in smaller and smallercircular paths. Under these conditions the full space between cathodeand yanode is not utilized and sensitivity is reduced. From theforegoing, it may readily be seen that for maximum sensitivity amagnetron ionization gauge should be operated `substantially at cut-offconditions. Cut-off conditions may be defined from Fig. 3 as themagnetic field, and cathode-anode voltage which cause the curve ofelectron curve versus magnetic field to decrease sharply from aninitially high constant value. In Fig. 3 this value is achieved atapproximately 380 oersteds.

From Fig. 3 it may further be seen that, at the point where maximumsensitivity is achieved, there is a substantial electron current flow.This means that a substantial number of electrons are impinging upon theanode at high velocity. This results in an inherent limitation in priorart magnetron ionization gauges. Most prior art magnetron ionizationgauges will not measure pressures lower than approximately -8 mm.pressure. This is because the very small positive ion currents whichflow at this level and below are masked by spurious background currents.One primary cause of these background currents is photoelectric emissionfrom the collector electrode. The photoelectric emission from thecollector electrode is caused primarily by bombardment of the collectorelectrode with soft X-ray photons which have their origin in the highenergy collisions of electrons with the anode. A further source ofphotoelectric emission at the collector electrode is the directimpingement of photons from the incandescent filament onto the collectorelectrode.

In the prior art, attempts have been made to decrease the minimummeasurable background current by decreasing the area of the collectorelectrode, thus decreasing the fraction of the total Isoft X-rays randvisible light striking the collector. This approach, however, has severelimitations, since decreasing the area of the collector electrodedecreases the sensitivity of the device and there is a finite limitbeyond which the area of the collector may not be decreased withoutsacrificing mechanical strength and causing microphonics.

In accord with the present invention I have discovered that it ispossible to greatly decrease the minimum measurable gas pressures inmagnetron ionization gauges by keeping the collector electrode at asubstantially large area, operating the device substantially at cut-offfor maximum sensitivity, and operating the cathode at la low the gauge.

temperature as compared with cathode temperatures conventionallyutilized to thus greatly decrease electron current from cathode toanode. With the electron current between cathode and anode greatlydecreased, the amount of soft X-rays emitted from the anode is alsogreatly decreased consequently reducing photoelectric emission from thecollector. I have found that in operating ionization gauges in thismanner, photoelectric emission by the collector ceases to 'be a limitingfactor in the minimum pressure which may be measured by the gauge. Ihave further found that operating the cathode at a low temperaturereduces the amount of incandescent light radiated from the cathode tothe collector electrode, further decreasing photoelectric emission fromthe collector. I also have found that the ratio of electron current tothe minimum measurable pressure in magnetron ionization gauges issubstantially a constant and may be expressed by the relationship whereIe is the electron current from anode to cathode in amperes, and P isthe pressure in mm.

By operating my ionization gauges at cathode-anode currents of 0.1 to1.0 microampere current, as opposed to the 5 milliampere currents atwhich ionization gauges are conventionally operated, I am able tomeasure pressures 10-4 to 105 of the minimum pressures measurable byconventional ionization gauges. These low cathodeanode currents may beattained for example by operating a 0.008 diameter, 11/2 long cathode ata tempera ture of approximately 1325 C. to 1450 C. as compared with thenormal temperature of l975 C. which would be conventionally utilized forthis cathode. The ionization gauges of the present invention operated inthis manner are operative to measure pressures as low as 5 X 10-13 mm.Minimum pressures measurable by most conventional ionization gauges areapproximately of the order of l0-8 rnm. of mercury. The prior artmodification discussed hereinbefore wherein the collector electrode isgreatly reduced in area by making the collector a thin wire, thusreducing the sensitivity of the device, is only able to measurepressures as low as approximately 5x10-11 mm. of mercury. It is evidenttherefore, that by operating magnetron ionization gauges lat cut-off toobtain maximum sensitivity and at a greatly reduced electron current toreduce photoelectric emission, facilitates the measurement thereby oflow pressures which have heretofore not been measurable with any otherpressure measuring device.

Other Structural features of thc devices constructed in laccord with thepresent invention may further be observed by referring again to Figs. land 2. .'n Fig. l, a further decrease in background currents which are alimiting factor upon the minimum measurable pressure is achieved byspacing ion collector electrodes 3 and 4 bctween annular metallic guardrings 11i-10' and 11-11 respectively. These guard rings are maintainedelectrically at the same potential as the ion collector electrodes. loncollector electrodes 3 and 4 are thus maintained free of leakagecurrents from anode cylinder 2 and cathode end plates 12 and 13. Endwalls for the device of Fig. l are provided by metallic disk-shaped endwall 12 and annular metallic end wall 13 which is integrally connectedwith a metallic tubulation 14 terminated in a flared end for easyconnection to a device or system, the pressure of which is to bemeasured by Filamentary longitudinal cathode 2 is held in spring tensionby a cathode spring 15 which bears between a cross-shape member 16, towhich one end of cathode 2 is connected, and U-shaped ibracket 17,integrally connected with annular end wall member 13. The opposite endof cathode 2 is permanently fastened in tension as for instance bybrazing or welding to a cathode terminal pin 18 which is press-fittedand brazed `into a central aperture in end wall 12. Because of thismethod of` support, cathode 2 may be located precisely at the axis ofanode cylinder 1 and maintained in this position in spite of mechanicaland thermal shock, greatly increasing the gauge sensitivity.

The respective electrodes of the device of Fig. 1 are all insulatinglyseparated from one another by annular ceramic insulating. members. Thus,annular insulating member 20 separates end wall 12 from guard ring 10',and insulating member 21 separates guard member 10 from collectorelectrode 3. Insulating member 22 separates collector electrode 3 fromguard ring 10 and insulating member 23 separates guard ring 10 fromanode support mem'ber 1'. Insulating member 24 separates anode supportmember 1 from guard ring 11, and insulating member 25 separates guardring 11 from collector electrode 4. Insulating member 26 separatescollector electrode 4 from guard ring 11' and insulating member 27separates guard ring 11' from annular end wall 13.

While metallic members 3, 4', 10, 10', 11, 11', l2, 13 and 18 may beconstructed of any highly electrically conductive material such ascopper, they are preferably fabricated from titanium because of theunique get-tering characteristics thereof. When these elements arefabricated from titanium the Iionization gauge of Fig. 1 may befabricated in accord with the method disclosed and claimed in mycopending application Serial No. 590,849 tiled concurrently herewith andassigned to the present assignee. In accord with the invention of mycopending application, electric discharge devices having titaniumelectrodes and titanium-matching ceramic bodies are fabrficated byringthese materials at a temperature of from 700 C. to^1100 C'. in aninert gas atmosphere. One of the advantages of this process is thattiring in an inert gas atmosphere prevents the accumulation of metallicleakage paths along the surface of the ceramic members and sharplyreduces leakage currents in devices fabricated in accord therewith. Theluse of titanium for these members thereby results in a great advantageover the yuse of other metals.

Insulating ceramic members 20, 21, 22, 23, 24, 25, 26 and 27 arefabricated from an insulating ceramic which closely approximates thethermal coeicient of expansion of titanium, thus facilitatingconstruction of the ydevices by tiring at high temperatures in 'accordwith my aforementioned copending application. One such type of ceramicis denominated as Forst-erite and comprises a sintered agglomerate ofsilicon oxide, magnesium oxide and aluminum oxide. One s-uch Forsteriteceramic and the method of preparation thereof are disclosed and claimedin the copending application of A. G. Pincus, Serial No. 546,215, tiledNovember 10, 1955, and assigned to the present assignee. In forming thedevice of Fig. 1 the outside diameter of the metallic members and of theinsulating ceramic members are chosen to be the same so that, uponfiring, the device is formed into an integral cylindrical unit which ishermetically sealed and which has a smooth cylindrical outer surface.Annular cylindrical magnet means 5 then tits over the cylindercomprising the body of the gauge and is insulatingly spaced therefrom bya suitable insulating sleeve 28 which may for example Ibe Teflon or anydielectric insulating material which will withstand moderately hightemperatures. Cathode 2, anode 1, and mesh collector 4 may convenientlyIbe made from tungsten, or molybdenum although other metals which do notreact violently with titanium at 7001100 C. are suitable.

In the device of Fig. 2, as in that of Fig. 1, cylindrical anode 6 issupported by an annular anode support member 30 which litsconcentrically over anode cylinder 6. Filamentary cathode 7 comprises avery closely bent V-shaped wire, conveniently of tungsten andconveniently approximately 0.008 to 0.01" in diameter. Cathode 7 extendssubstantially the entire length of anode cylinder 6 and is located atsubstantially the exact longitudinal center thereof. End4 walls for thedevice are provided 6 by annular metallic end wall members 31 and 32respectively. Filament 7 is supported by a pair of substantiallyL-shape'd tungsten members 33 which are in turn -supported by pins 34which extend through circular holes in annular end wall 31 and connectwith contact pins 35. This method of support for cathode 7, togetherwith the rigid manner in which anode 6 is supported keeps theseelectrodes concentric even with thermal and mechanical stress andincreases the tube sensitivity. Contact pin 35 are spaced from end wall31 by respective annular ceramic insulating members 36. Entrance intothe gauge is attained through tubulation 37 which is fastened integrallywith end wall 31 =by lbrazing. An annual metallic shield 38 is mountedupon support .pins 39 and prevents electrons from leaking out the end ofanode 6. Collector electrode 8 is disk-shaped and is suspended 'at oneend of anode cyly inder 6 perpendicular to the axis thereof fromcollector 5 field of approximately 160 oersteds.

support pin 40 which is suspended from collector terminal 41. Collectorterminal 41 is in turn supported fby elongated collector bushing 42which is in the form of a closed re-entrant annular cylinder having areduced annular portion 43 which protrudes through an aperture inannular end wall 32 making the leakage path thereover doubly re-entrant,thus reducing to a minimum, leakage currents which constitute a limit onthe low pressure performance limit of the gauge.

, Metallic members 30, 31, 32, 35 and 40 are preferably fabricated fromtitanium, while ceramic members 36, 42, 44 and 45 are preferablyconstructed of Forsterite ceramic which matches the thermal coelicientof expansion of titanium although other ceramics are suitable. With themetallic members of titanium and the ceramic members of Forsterite, thedevice of Fig. 2 may be constructed in accord with my aforementionedcopending application so as to provide an hermetically sealed envelopehavingH entrance only through tubulation 37 with a minimum of surfaceleakage over the interior parts thereof facilitating the attainment oflow pressure measurements.

The exterior of the device formed by end walls 31 and 32, annularceramic insulating side-wall members 44 and 45, and annular anodesupport member 30 is a smooth cylindrical surface over which annularcylindrical magnet means 9 is slidably movable and spaced therefrom by asuitable insulating sleeve 46 which may conveniently comprise any of thematerials utilized to form insulating sleeve 28 of the device of Fig. 1.

ln both of the devices of Figs. 1 and 2, operating potentials may besupplied from a source of unidirectional potential as for examplebattery 50 and regulated by potentiometer 51. Electrical current throughthe cathode may be supplied by a source of potential, eitherunidirectional or alternating, represented generally by battery 52, andcontrolled by rheostat 53. In operation, the anode is maintained at apositive potential of, for example, approximately 300 volts with respectto the cathode, and the collector is maintained at a negative potentialof, for example, approximately volts with respect to the cathode. Thevalue of the magnetic field necessary for operating the devices atcutoff conditions may vary with the dimensions of the device utilized,however, with a. device as illustrated in Fig. 1 operating at thepotentials indicated above wherein the anode cylinder is approximately5/x" long and 1/2" in diameter, and the collector electrodes areapproximately 5/1" in diameter, and spaced approximately y" away fromthe ends of anode cylinder 1, cutoff conditions are achieved with amagnetic field of approximately 300 oersteds. In a device as illustratedin Fig. 2 of the drawing wherein the anode cylinder is approximatelyll/s" long and 1" in diameter, the cathode has a length of approximately1% -and the collector electrode is approximately 1" in diameter and isspaced approximately 1A" away from the end of anode cylinder 6, thegauge operates at cutoff conditions with a magnetic Collector current,

measured by any conventional current indicating device, represented bymeter 54 is a direct indication of the measured gas pressure.

In addition to operating to measure heretofore unobtainable lowpressures, the devices of Figs. l and 2 possess the common features ofmaintaining extremely high leakage resistance between anode andcollector electrodes. In both devices, one important factor in obtainingthis high leakage resistance is the fabrication of the metal parts ofthe tube of titanium and the insulating ceramic members from atitanium-matching Forsterite ceramic, thus facilitating construction ofthe devices by tiring at a temperature of from 700 C. to 1100 C. in aninert gas atmosphere. When the devices are so constructed, the surfaceof the ceramic insulating materials remains completely uncontaminatedwith metallic deposits which are present when such devices areconstructed by other methods or red in other atmospheres. Additionalfeatures which aid in maintaining low leakage currents within the tubesare, in the device of Fig. 1, the presence of guard rings -10' and11-11' surrounding collector electrodes 3 and 4 respectively andoperated at collector potential to maintain no voltage across theceramics 21, 22, 25 and 26. In the device of Fig. 2 extremely highleakage resistance is maintained by the disclosed reentrant cylindricalcollector electrode bushing 42 which presents an extremely long leakagepath between anode and collector electrodes for the size of the deviceutilized.

Additionally, end-wall member 32 is connected at collector potential andfunctions as a guard ring between anode and collector, furtherdecreasing leakage currents.

Another important feature common to the devices of Figs. l and 2 is theparticular means which are utilized to maintain the filamentary cathodeat the precise longitudinal axis of the cylindrical anode electrode, andto keep this configuration even though the devices are subjected tointense mechanical and thermal shock. This is necessary in order tomaintain high sensitivity within the devices. As is mentionedhereinbefore with respect to the description of the curves of Fig. 3,when the magnetic field is increased beyond cutoff conditions, the pathsof the circulating electrons become much shorter due to a smallerdiameter of their helical paths. This shorter path results in fewerionizing collisions and reduces the sensitivity of the device. If themeans utilized in constructing the devices of Figs. l and 2 to maintainthe lamentary electrode at the exact longitudinal axis of thecylindrical anode were not utilized, electrons circulating in thecathode-anode space would not be equidistant from the anode on bothsides of the cathode while in their circulating path. Thus, in order toprevent excess cathodeto-anode current in accord with the presentinvention, the magnetic field would have to be increased to operate thedevice at cutoi, and the entire volume of the tube would not beutilized, thus causing the tube to operate at a reduced eiciency,lowering its sensitivity. In the devices of the present invention, thelongitudinal cathode is maintained at the precise center of thecylindrical anode, by rigidly fixing these electrodes in place byceramic-metal seals and pre-cut apertures, thus insuring maximumefficiency and sensitivity for the size of the device utilized.

An additional advantage of the devices` arises from the materialsutilized. Since the devices are constructed entirely of ceramic andmetal they may be used at extremely high temperatures up 'to 500 C. forexample. Additionally, since the metal parts of the tube are'constructed mainly of titanium, no additional getter need be provided.

While the invention has been set forth hereinbefore with respect tocertain embodiments thereof, many modiiications and changes willimmediately occur to those skilled in the art. Accordingly, by theappended claims I intend to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A magnetron ionization gauge electric discharge device for measuringgas pressures below l0*8 millimeters of mercury pressure comprising atherrnionic cathode, an anode, and a collector electrode disposed inclose juxtaposition to one another electric bias means for operatingsaid gauge under cutoff conditions; and means controlling the electricalcurrent supplied through said cathode at a rate to maintain electroncurrent between said cathode and said anode at a value of 0.1 to 1.0microamperes.

2. A magnetron ionization gauge electric discharge device for measuringgas pressures below 10"8 millimeters of mercury pressure comprising athermionic cathode, an anode and a collector electrode disposed in closejuxtaposition in said cathode; electron bias means maintaining saidanode at a positive potential with respect to said cathode; electricbias means maintaining said collector at a negative potential withrespect to said cathode; means impressing an axial magnetic eld throughsaid device perpendicular to the normal path of electrons from cathodeto anode, the magnitude of said cathode-anode potential and of saidmagnetic eld being correlated so that electrons emitted from the cathodejust fail to reach the anode and the device operates under cutoiconditions; and means controlling electrical current through the cathodeat a rate to maintain the cathode-anode operating rcurrent at a value of0.1 to 1.0 microampere.

3. A magnetron ionization gauge electric discharge device for measuringgas pressures below l0a millimeters of mercury pressure comprising athermionic cathode,

' an anode, and a collector electrode in close juxtaposition to saidcathode electric bias means maintaining said anode at a positivepotential with respect to said cathode; maintaining said collector at anegative potential with respect to said cathode; means impressing anaxial magnetic field through said device perpendicular to the normalpath of electrons from cathode to anode, the magnitude of saidcathode-anode potential and of said magnetic field being adjusted sothat electrons emitted from the cathode just fail to reach the anode,and the device operates under cutoif conditions; and means controllingelectrical current through the cathode so as to maintain the cathodetemperature at a value of approximately 1325 C.-l450 C.

4. A magnetron ionization gauge capable of measuring gas pressures belowl0*3 mm. of mercury comprising: an anode cylinder; an annular metallicanode support concentric with an external of a portion of said anodecylinder; a iilamentary cathode extending longitudinally along the axisof said anode, a disk-shaped metallic collector electrode insulatinglymounted within said device perpendicular to the axis of said anodecylinder and external of the volume thereof; a pair of metallic end wallmembers disposed in spaced relation at opposite ends of said gauge; ametallic tubulation integral with one of said end walls and formingmeans for attaching said gauge to a system, the pressure of which is tobe measured; a plurality of annular ceramic insulating membersinterposed between said end walls and said anode; support member andhermetrically sealed thereto to formA an evacuable envelope having asmooth cylindrical lateral wall, an annular magnet means slidablyinsertable over said envelope, and an insulating sleeve interposedbeforming an entrance to said device integral with one of said end wallmembers and substantially perpendicular thereto; a pair of metallicannular collector electrode guard rings in spaced relation with oppositesides of the peripheral portion of each of said collector electrodemembers; an axial lamentary cathode extending along the longitudinalaxis of said cylinder; a plurality of annular ceramic insulating membersinterposed between and hermetically sealed to :said metallic members andforming therewith an evacuable envelope having a smooth cylindricalexterior Wall; annular magnet means substantially coextensive with saidwall and exterior thereof; and an insulating sleeve interposed betweensaid magnet means and said cylindrical walls.

6. The device of claim 5 wherein said metallic members are of titanium.

7. A magnetron ionization gauge electron discharge device comprising: acylindrical anode; a metallic annular anode support member concentricwith and external of a portion of said anode electrode; a lamentaryelectrode extending longitudinally along the axis of said anodecylinder; a pair of metallic end wall members in spaced relation withone another and substantially perpendicular to the axis of said anodecylinder, a rst one of end walls having a tubulation integral therewithand a pair of metallic cathode support pins extending therethrough andin sulated therefrom, the second of `said end Wall member having acentral aperture therein; an elongated re-entrant ceramic collectorelectrode bushing extending through said aperture; a metallic collectorterminal member extending concentrically within the exterior end of saidbushing; a disk-shaped collector electrode suspended from said collectorterminal and spaced at one end of said anode cylinder substantiallyperpendicular to the axis thereof; a plurality of annular insulatingceramic members spaced between and forming hermetic seals with saidanode support member and said end wall members to form an evacuableenvelope having a smooth surfaced cylindrical exterior wall; annularmagnet means substantially coextensive with said wall and exteriorthereof; and an insulating sleeve interposed between said magnet meansand said cylindrical wall.

8. The device of claim 7 wherein said metallic mem4 bers are oftitanium.

References Cited in the file of this patent UNITED STATES PATENTS2,640,948 Burrill n June 2, 1953 2,745,059 Guager A May 8, 19562,750,560 Miles June 12, 1956 2,758,232 Fox Aug. 7, 1956 2,774,936 Becket al Dec. 18, 1956 2,817,030 Beck et al. Dec. 17, 1957

